Airway traffic control system



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31% 1W m. s. FIELD IRWY TRAFFIC CONTROL. SYSTEM 13 Sheets-Sheet l Filed Aug. 50, 1945 .GEL wlJ .NEL 1. :MF a@ wr www @mi @E D NNEE NMJNM :LE E56 /ww ,fw n JMU/Mw mi w ma 54h@ Q www m w ma@ ,l F :V Il |i| l IMHT l xl l1 JV l M V f l i i A ITJTI ||l li I: cmwlil V4||i |1 iwfli Iii 1t |11 l Il I# QV ATTORNEY i my, 1950 o. s. FlEgfD 2999,32

AIRWAY TRAFFIC CONTROL SYSTEM Filed Aug. so, 1945 l 13 sneeis-sne'ela 2 en "O i Uw May 30, 1950 o. s. FIELD 2,509,632

AIRWAY TRAFFIC CONTROL. SYSTEM Filed Aug. .'50, 1945 13 Sheets-Sheet 3 Feel@ Fan @A INI /EN TOR.

May 301, 1950 o. s. FIELD 2,509,632

AIRWAY TRAFFIC CONTROL SYSTEM Filed Aug. 30. 19.45 13 Sheets-Sheet 4 TAF4 WMM

ATTORN EY May 30 1950 `o. s. FIELD 2,509,632

AIRWAY TRAFFIC CONTROL SYSTEM Jg@ ATTORNEY May 30, 1950 o. s. FIELD 2,509,632

AIRWAY TRAFFIC CONTROL SYSTEM F'J'led Aug. 50, 1945 13 Sheets-Sheet 7 IN VEN TOR.

uw .ATTORNEY l May 3% 1950 o. s. FIELD IRWY TRAFFIC CONTROL SYSTEM @HGP mmv mi, W5@ i o. s. FIELD AIRWAY TRAFFIC CONTROL SYSTEM Filed Aug. 30, 15345 l5 Shee'ts-Sheet 9 IN VEN TOR.

//M/ ATTORNEY O. S. FIELD AIRWAY TRAFFIC CONTROL SYSTEM May 30, 1950 Filed Aug. 30. 1945 May 30, 1950 o. s. FIELD 50996.32

AIRWAY TRAFFIC CONTROL SYSTEM Filed Aug- 30 1945 w sheets-sheet m IN VEN TOR.

w ATTORNEY M50 c. s. FIELD 2,509,532

AIRWAY TRAFFIC CONTROL SYSTEM Filed Aug. 30, 1945 13 Sheets-Sheet l5 5, gu/d5,

Cttorneg Patented May 30, 1950 AIRWAY TRAFFIC CONTROL SYSTEM Oscar S. Field, Rochester, N. Y., assigncr to General Railway Signal Company, Rochester,

Application August 30, 1945, Serial No. 613,600

29 Claims. (Cl. 177-353) This invention relates to air traffic or airplane cab signalling systems and more particularly to a system of this kind Where the cab information is received by space radiation from a ground transmitter and is an improvement over the prior application of Field and Wight, Ser. No, 547,175, filed July 29, 1944, now Patent No. 2,458,361, dated January 4, 1949.

In an airplane cab signalling system of the type under consideration, if it is desired to divide a traffic route into block sections having a plurality of altitude routes over such traffic route, it is necessary to have a clear line of demarkation between successive blocks insofar as radio communication is concerned. That is, it is necessary to manifest in a particular block and on the ground the fact that there is an airplane flying in that block as well as the particular altitude at which it is iiying and furthermoreit is necessary for that airplane to communicate only with the ground located apparatus for that block so that the proper traflic condition is displayed in the cab of an airplane dying at that altitude in the block in the rear thereof. A

In view of the foregoing, and other important considerations, it is proposed to provide at the junction between each pair of successive blocks a radio transmitting fan marker which emits space radiation energy in a fan-like radiation pattern crosswise of the air route so that the reception of such space radiation on the airplane is a clear manifestation that such airplane is entering that block. It is proposed to use this energy on the airplane where it is characterized in accordance with the altitude at which the airplane is flying, to then transmit such characterized radio energy back to the ground station for that block where this energy is transformed by additionally superimposing thereon information characteristic of traffic conditions in advance of that block for the particular altitude at which` such airplane is flying. This space radiation energy is also distinctive for the particular block in which the airplane is flying especially insofar as several blocks both ahead and behind the airplane are concerned.

In view of the fact that the various kinds of distinctive characters that are to be transmitted by space radiation from a ground location to an airplane, and in the opposite direction, it is proposed to transmit such characters by either a synchronous type of distinctive control, which may for convenience be called the time of impulse method or a modulation type of distinctive control, or both. By the expression time-of-impulse is meant that successive cycles of time are each divided into a large number of impulse times, each impulse time of which may have ordinary energy or special energy of a character signifying a particular condition transmitted during such time, and by distinctive modulation is meant that either continuous energy or individual pulses of energy may be specially modulated to signify specific traic conditions.

As another object of the present invention it is proposed to use these two distinctive energy transmission features in different co-related manners by, for instance, transmitting analtitude character by radio energy on a particular time-of-pulse period and by transmitting information identifying a particular block by either the transmission of energy on a particular time- `of-pulse period or by modulating that energyat a particular frequency or both. In like manner it is proposed to transmit by space radiation the condition of traffic in one or more blocks in advance of a particular block to an airplane flying at a particular altitude in such block by either the time-of-impulse method or the modulation method or both.

As another object of the present invention it is proposed to allow a ground location to radiate space energy identifying a particular altitude and traffic conditions in advance thereof only if that ground station is then receiving radio energy identifying that same altitude from an airplane.

It is further proposed in accordance with the present invention to use the same radio beams that are transmitted from the first ground `location in the rear and the first ground location in advance of an airplane for the purpose of manifesting on such airplane Whether or not such airplane is flying directly over a route connecting such ground locations, that is, use the same space radiation for route designating and traffic condition manifesting purposes. It should be noted that the straight course finding portions of the complete system of this invention have been disclosed and claimed in my copending divisional application Ser. No. 123,845, filed October A2'?, 1949.

In accordance with another object of the present invention it is proposed to provide a suitable means for making a blind landing at an airfield when the airplane under consideration has completed its flight over a route or route portion.

`Other objects, purposes and characteristic features of the present invention will in part be described hereinafter and will in part be Vunderstood from the accompanying drawings, in `which l Figs. 1A, 1B and 1C illustrate an airplane cab signalling system of the type employing distinctive carrier frequencies, or distinctive modulations of carrier frequencies, for manifestation of the particular block an airplane is flying in and the particular traffic condition existing in that block and employing time-of-impulse characters for identifying the particular altitude in which the airplane is flying;

Fig. 1D illustrates a modified form of the invention illustrated in Figs. 1A, 1B and 1C;

Figs. 2A, 2B and 2C illustrate a system similar to that shown in Figs. 1A, 1B and 1C except that successive block identification is transmitted by the time-of-impulse method which time-ofimpulse method is also used for altitude identication and in which carrier frequency modulation is employed for transmitting distinctive conditions of traffic for one or more blocks in advance;

Figs. 3A and 3B illustrate a system wherein block identification is transmitted by carrier frequency modulation and wherein altitude identication and conditions of traiiic in advance are distinctively transmitted by the time-of-impulse method;

, Fig. 4 illustrates a system of route flying by the use of which a pilot may fly a straight line between two, successive ground radio transmitting stations irrespective of a substantial tendency for the airplane to drift sidewise due to a side wind;

Figs. 5A, 5B and 5C illustrate a system for blind landing the airplane when it has reached the end of the route over which it is to travel; and

Fig. 6 illustrates a typical multiple anode tube as shown in Pat. No. 2,448,487, dated August 31, 1948, which tube is contemplated as employed as a part of this disclosure.

Structure-Figs. 1A, 1B and 1C.-The system illustrated in Figs. lA-lC of the drawings is provided to indicate in the cab or cockpit of an airplane the conditions of traffic ahead of such airplane for the particular altitude at which such airplane is flying and to manifest at a ground location for that block section the fact that such airplane is dying in that block section and is flying at a particular altitude. In other words, in order to manifest traffic conditions in advance in the cab of an airplane ying at a particular altitude provision must be made to manifest the presence of all airplanes travelling in such and adjacent block sections and at such altitude. In order to manifest the presence of an airplane .precisely within the limits of a particular block `section on an air traffic route it is necessary that the beginning and end of such block section be rather definitely defined. In accordance with the present invention and particularly as illustrated in Figs. 1A, 1B and 1C (also Figs. 2A, 2B, 2C, 3A and 3B) it is proposed to manifest the starting .point of a block section by the provision of a .at the beginning of the next block section. It

is further proposed to use distinctive space radiation energies in successive blocks repeated at intervals so that the reception of space radiation energy 0f a particular character is a clear manifestation of traiiic conditions in advance for that block, such distinctive space radiation energies .zbeing transmitted only in blocks which are so far apart that the energy received from the nearest block transmitting energy of that character, namely, the block in which the airplane is travelling will dominate the energy of like character emitted from the next block in advance which emits energy of the same character.

Referring to Fig. 1A two block sections one extending from x l to fix 2 and the second block section extending from fix 2 to x 3 have been shown. In accordance With the specific embodiment of the invention shown in Figs. lA-IC, it is proposed to use distinctive radio carrier frequencies for the various distinctive space radiation transmissions to define blocks and to transmit this space radiation energy in successive cycles of pulses wherein a particular pulse in a cycle characterized the particular altitude under consideration. It is further proposed to modulate the carrier frequency to manifest traffic conditions. Y .l Y

Referring to Fig. 1A the radio carrier frequencies for the three fan markers illustrated at fixes l, 2 and 3 respectively are frequencies Fl, F2 and F3, whereas the radio frequency transmitted from the fix in advance of such fix to'V an airplane fiying in the block section in advance of each of these fixes have been vdesignated Fl l, FIZ and F13 (not shown) respectively. Similarly, the return space radiation energy that is transmitted from the airplane back to the next fix in the rear of that block have carrier frequencies of values F2I, F22 and F23 respectively. VIn other words, carrier frequencies FI, FH and F2I are used in the block section fix l-fix 2, carrier frequencies F2, F12 and F22 are used inthe block section extending from nx 2 to fix 3 and carrier frequencies F3, FI3 (not shown) and F23 are used in block sections extending from fix V3 to x 4 (not shown). After the fourth block` (only partly shown) which employs frequencies F4 (not shown), FI4 and F24 (not shown) these frequencies are repeated. Obviously, this repeat of frequencies may occur after any desired number of blocks.

In this connection, it may be pointed out that when an airplane has once entered a particular block section space radiation energy is transmitted from the ground at the exit end of such block section to the airplane modulated to a socalled pick-up frequency but not to characterize traic conditions in advance for a particular altitude. This space radiation energy when received on the airplane has the modulation character dening traffic conditions then deleted therefrom after which it is, on the particular pulse time identifying the altitude at which the airplane is then flying, retransmitted at a different carrier frequency to the fix at the entrance end of that block section from whence this pulse is again transmitted to the x at the exit end of such block section at which point the radio pulse is then modulated to characterize trac conditions in advance. In this way the fact that the airplane is manifesting its presence is repeated back to the pilot of that airplane so that he is aware of the fact that he is properly protected against being struck by an airplane from the rear which is equipped with similar cab signalling apparatus. In other Words, the reception of a signal on a particular airplane is a definite manifestation that that particular airplane is manifesting its presence at a Aparticular altitude in a particular block section.

Referring now to Fig. 1B of the drawings-fit willbe observed that the ground located appara- `tus for fix 2 has been illustrated conventionally in full. This ground apparatus includes suitable vmeans for transmitting a synchronizing alternating current, say of 60 cycles, as well as for transmitting trafc conditions existing in advance for each altitude. In the particular embodiment of the invention illustrated, conditions of trafc at one fix are transmitted to the rst and second fix in the rear through the medium of line wires 21 and 28 whereas the synchronizing frequency is transmitted over line wires 25 and 26.

The present invention, as above pointed out, contemplates transmitting distinctive altitude information by the time-of-impulse or synchronous method and for this reason rotating beam electron tubes RBETI and RBETZ have been provided as shown in Fig. 1B of the drawings and rotating beam electron tubes RBET3 and RBET4 have been provided on the airplane as illustrated in Fig. 1C of the drawings. The rotating electron beams in all of these rotating beam tubes are kept in synchronism through the medium of the synchronizing frequency transmitted from fix to x and from the various fixes to the various airplanes through suitable means including the transmission lines 25 and 26 and space radiation apparatus more specifically described hereinafter. As an example, these beams may rotate at a speed of 60 revolutions per second as will be the case if 60 cycle energy is used and these tubes are preferably of a construction such as more fully described in the prior application of Field and Wight, Ser. No. 599,328, filed June 14, 1945, now Pat. No. 2,448,487, dated August 31, 1948.

The various rotating beam electron tubes have been shown in their simplest form, certain connections thereto being abbreviated, but the complete connections are shown in Fig. 6 as made to a tube of the structure and type disclosed in the above noted Patent No. 2,448,487. Referring to this Fig. 6, it will be seen that the typical rotating electron beam tube RBET2 for example, comprises a cylindrical envelope 315, preferably constructed of glass, which is evacuated and contains the electron scanning apparatus. This glass envelope is surrounded by a laminated iron core 31E containing one or more windings 311, as the case may be, to produce a two-pole rotating magnetic field. Although this core 316 and 311 has been illustrated as constituting a field structure of the gramme-ring type, it may be, if desired, a toothed stator having inwardly directed magnetic teeth into which a two-pole two-phase lap winding may be contained. In order to produce the necessary rotating magnetic eld for the eld structure shown, a source of polyphase energy, conventionally illustrated as two-phase energy, is applied to the winding 311. Obviously, any multi-phase, such as three or six phase, winding and a corresponding phase source may be used, if desired.

The two-phase energy is illustrated as being derived from transformers Tr4 and Tr5 having their primary windings from an alternating current source, such as the line wires 25 and 26 constituting a 60 cycle line circuit. Suitable means is provided between the alternating current line and the primary windings of the transformers to provide a proper displacement in phase between the two transformers. This phase displacement is preferably obtained through the medium of a condenser C included in the circuit of the primary winding for the transformer T14, and a choke coil or inductive reactance IR ineluded in the primary winding of the transformer Tr5. The condenser C and the inductive reactance IR have such capacity and inductance respectively that each causes a phase displacement of substantially 45. It is readily `understood that by connecting the output terminals of the first set of secondary windings to respectively opposite points on the winding 311 and by having these secondary outputs displaced by with respect to each other a twophase rotating magnetic field will be set up in the evacuated tube or envelope 3 I 5. .Y

Within the evacuated tube 315 and arranged in the form of a cylinder are provided four suppression grid elements 324, 325, 326 and 321. The suppressor grid elements 324 and 326 are connected to the opposite ends of the added secondary winding of the transformer Tr4 whereas, the suppressor grid elements 325 and 321 are connected to opposite ends of the additional secondary winding of the transformer Tr5. Around the outside of the tube structure are the plates Pa, Pb, Pc, etc. each of which have considerable length and area. It will be seen that the various plates Pa, Pb, etc. are each located back of an opening or window in or between the suppressor grids 324-321. Between the plates Pa, Pb, etc. and the suppressor grid elements is another suppressor grid 331 which is directly connected to the cathode Ca. The cathode Ca is in turn surrounded by the control grid g which is connected externally to the controlling potential which is illustrated as being the receiver R-D21 by way of example. The suppressor grid 331 is used to suppress the effect of secondary emission from the plates or anodes Pa, Pb, etc. and its connection to the cathode is through the medium of conductor 338 in accordance with the usual practice.

It is believed that the above brief description of the tube structure will suffice, since further detailed description may be found in the patent above referred to.

In the specific embodiments of the invention illustrated it is assumed that airplanes will only ily at four different altitudes of 1000, 2000, 3000 and 4000 feet respectively, although many more altitudes may be resorted to, if desired. The presence of an airplane at a particular altitude in a particular block section is made manifest through the medium of four approach relays ARI ARZ, ARS and AR4 at the exit x of such block, these relays having only been illustrated for the four altitudes at x 2 (Fig. 1B). These various approach relays AR control various home relays HRI, I-IR2, HRS or HR4 and various distance relays DRI, DRZ, DR3 and DR4 where the numeral suffix identifies the particular altitude with which such approach relay, home relay or distant relay is associated. Each x is provided with suitable antennas for transmitting concentrated beam space radiation energy in various directions. For instance, a fan marker has been shown at each of the fixes 1, 2 and 3 and the antennas thereof have been designated TAFI, TAFZ and TAF3 respectively, these antennas being energized by radio transmitters TF1, TF2 and TF3, respectively. Similarly, each block is provided with a transmitting antenna TA and with a receiving antenna RA. The transmitting antenna is associated with the exit x whereas the receiving antenna is associated with the entrance fix. The transmitting antennas for fixes I, 2 and 3 respectively have been designated TAF14, TAF! 1, TAF|2 whereas the receiving an,

7 ltennashave been designated KRAFZI, `RAF22' and -RfAF23.:y Thesetransmitting..antennas aretfrespectivelyvconnected to transmitters TFM, TFII and. TFIZ (some of which have been omitted from the drawing) whereas the receiving anf 'tennasare connected to radio Vreceivers and` detectors R-DZI, R-D22 and Rf-D23 respectively. Each of these radio transmitters, such as radio'transmitter TFI I for fix 2,is provided 'with input tone generators for modulating the radio tcarrierV frequency, these tone generators having been designated TG-PU, 'FG-R, TG-.Y and jTGr---CY It may be pointed out that these modulating frequencies characterize respectively a .pick-up modulation PU, an R or danger modula- Vtiony'a Yor caution modulation, and a G or clear modulation. In addition to one of these modulating frequencies the carrier frequency is Valso'niodulated `to the synchronizing frequency which is assumed to be 60 cycles in the particular embodiments of the invention illustrated, as ,conventionally shown by wires 32 and 34 (Fig. 1B) i .Referring now more particularly to the air- -plane carried apparatus illustrated' in Fig. 1C of the drawings it will be observed that three separate antennas TA, FanRA and RA are fastenedto the lower part, of the `airplaneAPZ (see Figs. 1C and 1A)the antennasv TA andRA beingy employed for respectively transmitting and 'receiving space radiation energy for traiic manifestation purposes whereas the antenna FanRA is employed to receive space radiation energy -from the fan marker at the entrance end to a `particular block section to manifest entrance of the airplane into a new block. Since different radio carrier frequencies are employed in the system shown in Figs. 1A, 1B and 1C foreach of four blocks in succession along the air route each' of these antennas must be capable of transmitting or receiving, as the case may be, space radiation energy of four different fre'- quencies. It should of course be understood that the invention is not limited to the employment of repeating of specific carrier frequencies at any specific number of fixes along theroute `but for convenience the invention has been disclosed as having these frequencies repeated-for everyiifth Axrfalong theroute. Also, if desired, instead of `using different carrier frequencies for like antennas in successive blocks, like'carrier` frequenciesdstinctively modulated may be employed. Since four different kinds of carrierfrequencies are assumed to be employed it will be Vobserved that the fan marker receiving antenna FanRA is directly connected to'four different receivers RFI, BF2; RF3 and RF4 whereas the radio receiving antenna RA is connected to receivers and `detectors R-DI I, R-DI2, R-DI3 and Etf-Dld. `'The received energy in each case flowsY to the particular receiver or receiver-detector which is tuned tov agree with the space radiation energy received. Similarly, the transmitting antenna ATA has energy supplied'thereto from any one Aof four different radio transmitters TF2I,'TF22, TF23 and TF24.

' As shown the output energies from the radio receivers RFI, RF2, RFS and RF4 are supplied to the upper windings of the repeater Arelays PRII, PRI2, PRIS and PRI4. Obviously only one of these receivers is active at any one time. -The iiow of this energy is, however, dependent upon the associated pick-up relay PURI I, PURIZ, PUR3 or PURM being in its picked-up condition. These pick-up relays PURI I, PURIZ,

PUT-M3 .candyPURM are energized from tuned transformers TrII, TrI2, TrI3 and TTM, re'- spectively; Vwhich transformers receive their energies from the receiver-detectors R-DI I, R-DI-2, R-Dl3 yand Etf-DM respectively. It will iba-'observed that each of these receivers R--DILv R---DI2, R-Dl3 and R-DI4 has associated therewith a 60 cycle filtering unit GUFII, BUFIZ, BBFIS and EFM respectively and that the energy from any one of these lters'may flow to the 60 cycle Vrelay 60 and also to the Windings 23 and-24 ofthe rotating beam electron tubes RBET3 and RBET4 respectively. Corresponding windings of the rotating beam tube RBETI and RBETZ (Fig. 1B) have been designated'ZI and 22 respectively. It will be observed that-the output Aenergy fromthe rotatingbeam electronv tube RBETS may activate the grid y of the rotating beam electron tube RBET4 through the medium -of a circuit including a modulation filter MF. This filter is employed to transmit pulses, one per cycle of synchronizing energy, to the grid o-f the rotating beam electronv tube RBET4. These pulses have the particular modulating frequency, which was originally transmitted by such pulses, removed therefrom, so that as these pulses are rfa-transmitted to the ground location they no longer manifest trafc conditions, but merely manifest the altitude by the time of their occurrance. In this connection, it shouldrbe understood that the modulation lter MF (Fig. 1C) includes suitable voltage reversing means ,such as disclosed in said prior application of VField and Wight, Ser.V No. 599,328 so that positive not negative impulses are applied to the grid g of tube RBETI. It will also be seen that the output energy from the rotating beam electron tube RBET3 may pass from one of therplates I, 2, 3 or 4 thereof depending upon the position assumed by the altimeter controlled contact 33 to one of the tuned transformers TrR, TrY or TTG depending upon the modulating frequency modulating that pulse and that this energy may be rectified through the medium of the full--wave rectiers illustrated from whence it may pass to the particular danger relay RR, caution relay YR, or clear relay GR, respectively, as the case may be. These 4relays RR, YR and GR control red, yellow and green indicating lamps R, Y and G respectively.

i Operation-Figs. `1A, 1B and 1C.*Referring tovFig.- 1A let usH assume that the airplane AF2 has not yet passed the fan marker antenna TAFI emitting la radio carrier frequency FI and located atthe entrance to the block section fix I *fix 2 is flying at an altitude of 3000 feet. Let us now assume that this airplane AF2 has advanced to the point Where it receives radio energy of carrier frequency FI and that it also receives radio energy of carrier frequency FI i modulated at modulation frequency PU' from the radio transmitting `antenna TAFI I located at fix 2.' Referring now to Fig. 1C of the drawings the space radiation energy Vreceived bythe antenna RA may flow over wire 35 to the radio" receiver and detector R'-DII from whence Aa modulating frequency conveniently Vcalled the pick-up modulation may flow through the tuned transformer TrII and may then ow through the vassociated full-wave rectifier to the pick-up relay PURi I as direct current. This will `cause the relay PURII te pickup and close its contact 35. With contact 36 closed the fan `marker radio energy received by the fan marker receiverRFl may ow from the terminal plus Y(-}-), receiver RFI, the upper Winding'of repeater relay PRII through contact 36 of relay PURII to the other terminal of this source of current supplying plate energy to the receiver RFI. The flow of this energy causes the repeater relay PRI I to pick up and stick up and in so doing allows a pulse of current to be generated :by the radio transmitter TF2I of carrier frequency FZI on the third pulse of the cycle of the rotating beam electron tube RBET4. The circuit for this energy may be traced from the terminal (I-) of a suitable source of current, wire 3T, radio transmitter TFZI, wire 38, front contact 30 of the repeater relay PRI I, wire 40, valtimeter controlled contact 4I assuming the 3000 foot position, plate 3 of the rotating beam electron tube RBET4, through the electron beam thereof to the cathode Ca of this electron tube RBET4 to the other terminal of said source of current. The grid g of this rotating electron tube RBET4 is active at this time (third impulse time) even though approach relay AR3 (Fig. 1B) is still down and transmits a pick-up pulse PU during this third impulse time of a beam rotating cycle of the rotating beam electron tube RBETI. The flow of this impulse of current during the third pulse of the cycle defined by the rotating beam electron tube RBET4 to the transmitter TFZI (Fig. 1C) will cause a pulse of space radiation energy to be transmitted through the medium of the transmit` ting antenna TA on the airplane to the ground located receiving antenna RAFZI (Figs. 1A and 1B), this space radiation impulse being a radio frequency FZI. This energy is then received and detected by the radio receiver-detector R D2I from whence it may ilow through Wire 42 to the grid g of the rotating beam electron tube RBET2. This will of course cause activation of this grid g of this tube at the time when its electron beam wipes past the plate 3. Current will therefore flow at this instant from the terminal of a suitable source of current, wire 44, winding of the approach relay AR3, Wire 45, pl-ate 3 of the rotating beam electron tube RBETZ, through the electron beam of this tube, to the cathode Ca to the other terminal of this source of current. It is of course understood that the approach relay AR3 has not yet picked up and that the energy that was transmitted to the airplane receiver antenna RA and which picked up the pickup relay PUR! I could not have been derived from the electron tube RBETE (Fig. 1B) and that this energy Was instead derived from the electron tube RBETI. This current flowed through a circuit which may be traced from the terminal of a suitable source of current, radio transmitter TFI I, the pick-up portion PU of the tone generator TG-PU, wire 41, back contract 48 of the approach relay AR3, Wire 49, plate 3 of the rotating beam electron tube RBETI, to the cathode Ca of this tube to the other terminal of this source of current. In o-ther words, so long as the approach relay AR3 remains deenergized the radio transmitter TFi i receives its energy from the rotating beam electron tube RBETI during the third pulse of a cycle of radio pulse communication but that this energy is derived from the rotating beam electron tube RBETZ as soon as the approach relay AR3 picks up. In other words, during the rst one or more cycles of spaced radiation impulse transmission on the third pulse time of such transmission is derived `from the rotating beam electron tube RBET I and is transmitted to the airplane APZ (Fig. 1C) because this airplane is flying the 3000 foot altitude. This energy is modulated to pick-up modulation frequency PU. This energy is then re-transmitted to the grid g of the electron tube RBETZ (Fig.

1B) to cause it to try to pick up the approach relay AR3. So long as this approach relay AR3 does not pick up the electron tube RBETI transmits newly originated radio energy up to 'the airplane which is then repeated down to the ground. However, as soon as the approach relay AR3 picks up the rotating beam electron tube RBETI no longer serves a useful -purpose insofar as pulse 3 of the impulse cycle is concerned but may still perform a useful function insofar as pulses I, 2 and 4 of this impulse cycle is concerned. Also, this energy is modulated to pick-up frequency so long as approach relay AR3 remains deenergized but when relay AR3 picks up it is modulated to a clear, caution or danger frequency G, Y or R respectively dependent on tramo conditions in ad- Vance.

It should be understood that relay PRII (Fig. 1C) if once picked up will be held energized by a holding circuit which may be traced from the terminal of a suitable source of current, push button contact 10, front contact 'II of relay PRI I, back Contact l2 of relay PRIZ, back contact 'I3 of relay PRI3, back contact 'I4 of relay PRI4, stick contact 'I5 of relay PRI I, lower Winding of this relay PRI! to the other terminal of this source. The relays PRIZ, PRI 3 and PRI4 may ibe stuck up by similar holding circuits including stick contacts 16, TI and 18, respectively. The subsequent picking up of any one of the relays PRI2, PRI3 or PRN will result in the dropping of relay PRII for obvious reasons.

Let us now assume that the approach relay AR3 has received a suicient number of impulses to have picked up and has opened its back contact 48 and closed its front contact 50. These contacts are preferably adjusted make-before-break as conventionally illustrated by the letters MB. Energy may now flow through the electron tube RBETZ from the terminal of a suitable source of current, to radio transmitter TF I I, tone generating element G of the tone generator TG, wire 5 I, front contact 52 of the home relay HR3, front contact 53 of the distant relay DR3 (traliic conditions in advance being assumed to be clear), front contact 50 of the approach relay AR3, Wire 45, plate 3 of the tube RBETZ, the rotating electron beam thereof to the cathode Ca and then to the terminal of this same source of current. The flow of current in this circuit causes the radio transmitter TFII to transmit space radiation energy of carrier frequency FII modulated to both frequency G and 60 cycles the first of which signies clear traic conditions. The 60 cycle modulation is brought about through the wires 32 and 34 connecting transmitter TFII to the transmision line 25-26. As this space radiation energy is received by the receiving antenna RA carried by the airplane (Fig. 1C) this energy may ow over Wire 35 to the receiver-detector R DII and from whence potential of current modulated at frequency G may be applied through front contact 55 of repeater relay RPI I, wire 56, front contact 51 of the relay 60, wire 58 to the grid g of the rotating beam electron tube RBET3 at the instant its electron beam Wipes over plate 3 of this tube. With the grid g of this tube RBET3 now activated current may now from the terminal of a suitable source of current, primary Winding of the tuned transformer TTG, push button contact 29, altimeter controlled contact 33 assuming the 3000 foot altitude position, plate 3, e-lectron beam of tube RBET3 to the cathode Ca of this same tube to the other terminal of this same source of current. This current is of course mod ulated at the clear, green or G frequency and flows freelywthrough thef transformer TTG, and1 causes Athe signal relayGR to-pick up. The current flowing in the relay GR having in the mean--j transmitting rotatingbearn electron tube RBETli.

This will in turn cause an answer-back space radiation impulseto bek transmitted to fix I (Fig. 1B)

by transmitter TF2! (Fig. 1C) and its associated antenna TA. It is thus seen that each impulse of space radiation energy that is transmitted from the'exit end of a block section to` an airplane is usedg'not only for producing the proper signal indication in the cabof the airplane :but that this impulse, -Y after having the modulation filtered therefrom, is again transmitted to the ground location to trigger off, so to speak, the next radio impulse which is to'be transmitted tc the airplane after it has first been modulated to characterize traffic conditions then existing for that altitude for that block.

1n this connection it should be observed that the home relay HR having a suffix corresponding tothe 'altitude under consideration, is energized so; long as theapproach relay for that altitude' only energized if the approach relay for that altitude at boththe first fix and the second x in' advance thereof assume their 'deenergized position. The circuit for the'horne relay HRI for'x 2 may be traced in Fig. 1B of the drawings from the" terminal of a suitable source "of current, back contact 52 of approach relay ARI' at fix 3, wire 2l, winding of the relay HRI at fixv 2to the other terminal of this same source of current. Also, the energizing circuit for the distant relay DRI for x I may be traced from the terminal of this same battery through this same back contact 62 of relay ARI for x 3, wire 21, back contact 63 of relay ARI forfix 2, wire 28,'winding of the relay DRI for iix I, to the other terminal of this same source of current. It is thus seen that a home relay HR for any x is energized provided the approach relay for the altitude at the next nx in advance is deenergized and that the distant relay DR`for any particular fix and altitude is energized 'only Vif' the approach relays AR for both that altitude of the next two xes in ad# Vance are deenergized. The relays AR2, ARS andV AR4 are provided with similar control circuits.

Let us now assume that the airplane AP2 (Fig. 1A) proceeds through the block section extending from fix I to fix 2, and passes over fix 2. As this airplane AP2 passes over x 2 space radiation energy of carrier frequency F2 is received byA thefan marker antenna FanRA at the same instant Vwhen the radio antenna RA on this airplane AP2 receives radio energy of carrier frequency FIZ. For reasons heretofore described the reception of these radio energies of frecircuitifor the repeater relay -PRI I f to` therebyl planeAP2which is modulated to manifest traflc l conditionseXisting-in advance for that altitude.

Referringto Fig..1.C, if for any reason the pilot..

wishes to drop. allofthe relays PRI I, PRI2, PRI3 and PRIII to theirdeenergized position because,

for instance, he is shifting from one route tor another and wishes to have a new relay to pick up 0r the samerelayv .to again pick up, he .lmayY depress the push button 10. This opens the. stick circuit for. any one of the relays PRI I -PRN that may then be `assumingit energized position. As already pointed out if a second relay PR is picked up when anotherV one of these relays PR is in an' energized condition this picking up of a second` relay PR.. causes deenergization of the formerly: energized relay. Obviously, when traffic condi-"2" tions in advance for the altitude at which an airplaneis, flying change the carrier frequency will 1immediatelybe modulated at the new rate de;w y

fining the new traffic condition.

There may be times when -a pilot desires'to ascend or descend to a different altitude; In' order for VVhim to observe trai-lic conditions in ad;-` va'n'in' an altitude different from the one in" which the airplane is then flying, that is, if the pilot wants to look into a different altitude, the

pilot may set his selecting hand switch 65 (Fig. 1C) .to the position corresponding to the altitude into which he wishes to look to observe traflic' conditions in advance. For instance, under the conditions illustrated in the Fig. 1C the airplane is assumed to be flying at an altitude 3000 because the two altimeter controlled switches 33 and IH assume the altitude 3 position. If now the "'pilot wishes to look into altitude 2 he may operate his hand switch to the second position from the right and may then depress the push button 29. This will cause the cab signals to indicate traflic conditions in altitude 2 whereas the presence of the airplane in altitude 3 is manifested on the ground. The latter conclusion is correct because the grid G of the rotating beam electron tube RBET4 is still connected to plate 3 of tube RBET3 through the medium of the modulation filter MF and the altimeter switch 33, and furthermore the altimeter switch 4I still assumes its altitude 3 position and transmits radio energy through the medium of the transmitter antenna TA to the ground located apparatus having a time-of-impulse character which manifests that upon looking into another altitude observes eitherv clear (green), caution (yellow) or danger (red) space radiation energy emitted he is informed quencies F2 a 1' 1 `d FI2 will cause the pick-up relay,

PURI2 and the repeater relay PRI2 (Fig. 1C) to'assume their energized positions. The picking up of relay PRI2 will by the momentary opening of back contact l2 interrupt the holding or stick that the same block in which he is flying is occupied by another airplane ying at the altitude into which he is looking. Under this condition it is of course unsafe for him to enter such other altitude. If, however, the pilot receives neither a clear, caution or danger indication he is reasonably assured that the same block at such other altitude is not occupied and that he may enter suchother altitude. In this connection a fourthl or approach indication light similar to indicating lights G, Y and R may be provided, this light being controlled through a tuned unit tuned to the pick-up frequency PU. This fourth light could then be used only when the push button 29 is depressed and could be used to indicate that the same block of the altitude he is looking into is vacant.

ModiJication-Figlll-Instead of having space radiation energy transmitted rearwardly downwardly from an airplane, as `illustrated in Figs. 1A and 1C of the drawings, this radio energy may be transmitted forwardly and downwardly as illustrated in Fig. 1D of the drawings. By resorting to this construction the radio energy when transmitted from the :airplane to the ground equipment reaches the next fix in advance of the airplane and for this reason it is unnecessary to transmit the information transmitted from the airplane to the ground from one fix to another in the direction of airplane travel, but in this case this energy `which also manifests block altitude occupancy must be transmitted rear.- wardly from the exit x to the entrance nx for that block. It would therefore seem that this modified construction shown in Fig. 1D of the drawings is neither inferior nor superior to that of the construction shown in Fig. 1A of the drawings from a line wire standpoint. There may, however, be other reasons for preferring one form over the other.

Structure-Figs, 2A, 2B and 2C.-Referring to Fig. 2B of the drawings it should be understood that the home relays HR and distant relays DR are controlled in exactly the same manner as correspondingly designated relays illustrated and described in connection with Fig. 1B of the drawings are controlled. Referring to Fig. 2A of the drawings it will be observed that the structure shown in 2A is the same as that illustrated in Fig. lA except that the two transmitting antennas TAFI and TAFII at each of the fixes transmit the same two frequencies FI and Fil respectively whereas in the Fig. lA structure the fan marker antennas at four successive xes each transmit a diiferent frequency after which these transmitting frequencies are repeated in sequence for the next four '.xes, and so on. Similarly, in the Fig. 2A structure the receiving antennas RAFZ! and their associated receivers located at each of fixes l, 2 and 3 are constructed to receive the same carrier frequency F2! whereas in the Fig. 1A structure each four successive receivers will be tuned to a different carrier frequency. The speeic structure of the ground located apparatus is also diiferent as is evident from comparing Fig. 2B with 1B. In Fig. 2B of the drawings the same carrier frequencies are used in successive blocks but these successive blocks transmit their space radiation energies at different time-of-impulse periods. This .is evident from the fact that ancdes 2 of tubes RBET5 and RBET@ are used in block nx I-nx 2 whereas anodes 3 are used in block fix Z-iix 3. This is done in order to be sure that an airplane flying in a particular block will not receive energy intended for an adjacent block.

Again referring to Fig. 2B of the drawings additional rotating beam electron tubes RBET5 and RBET are employed the electron beams of which.

rotate preferablyT at a speed of 6i) revolutions per second. These eiectron tubes RBETE and RBETS are connected in tandem or series with the two electron beam tubes RBETZ and RBETI, respectively. In this connection it should be observed 14 that the electron beams of the tubes RBETI and RBETZ are rotated at four times the frequency of rotation of the electron beams for tubes RBET5 and RBET9. This is accomplished by employing a frequency quadruplicator FQ. In this connection it is desired to point out that the rotating beam electron tubes RBETS and RBET9 charac-` terize the impulses to identify the particular block for which the emitted space radiation energy is intended, whereas the electron beam tubes RBETI and RBETZ identify the particular altitude for the particular block under consideration. These impulses of space radiation energy are further characterized by having their carrier frequencies modulated to manifest traiiic conditions in advance for that block-altitude and the circuits for accomplishing this are substantially the same as those already described in connection with Fig. 1B and will for convenience have the devices and contacts thereof identied by like reference characters.

Comparing now the airplane carried apparatus illustrated in Fig. 2C with that illustrated in Fig. 1C it will be observed that the apparatus on the extreme right-hand side of thedrawings is exactly the same as that of Fig. 1C but that the apparatus in the left-hand portion of Fig. 2C is somewhat different. The differeneces between the two apparatuses are the result of providing means for identifying successive blocks by the time-of-impulse method as distinguished from the frequency modulating method employed in the Fig. 1C structure. Referring to Fig.. 2C it will be observed that the airplane carried transmitter TF2I transmits only frequency F2I whereas the airplane carried transmitters TFZI, TFZZ,

TF23 and TFM of Fig. 1C transmit frequencies F2I, F22, F23 and F20., respectively. Also, whereas in the Fig. 2C structure, the radio receiver R-DII connected to the antenna RA by a wire 88 is capable of receiving only frequency FII while corresponding receivers R-DII, RDI2, R-DI3 and R-Dll` in the Fig. 1C structure are capable of receiving distinctively carrier frequencies FI I, FIZ, FIS and FM respectively. Similarly, the fan space radiation receiving :antenna FzmRA illustrated in Fig. 2C of the drawings received energy only for one receiver capable of receiving frequency FI Whereas the corresponding antenna of the Fig. 1C structure is connected to four radio receivers RFI, RFZ, RFS and RF4.

As already pointed out the repeater relays PRI I, PRI2, PRIS and PRN of Fig. 1C are picked up respectively only if radio energies of proper frequencies are received by both of two receiving antennas FanRA and RA whereas in the Fig. 2C structure radio energies must be received on the same and proper time-of-impulse period by each of the receiving antennas FanRA and RA. As already described in connection with the Fig. 1C structure these space radiation energies for picking up one of the repeater relays PRI l PRIZ, etc. are detected by suitable tuning and filtering apparatus whereas in the Fig. 2C structure these energies are detected through the medium of both,

tuned circuits and rotating beam electron tubes RBETi and RBET'I and associated discriminat` ing pickup relays PURI, PURE, PUR3 and PURII. These relays PURi, PURZ, PURB and PUR4 control the repeater relays PRI PR2, FR3 and PRA.

respectively. These rotating beam electron tubes RBETG and RBETl have their rotating beams roe.

tate at the comparatively low speed of say 60` revolutions per second as is also true of the rotating beam electron tube RBETB whereas the 

